1. Field of the Invention
The present invention relates to the field of image sensing devices. More specifically, the present invention relates to a photodiode Complementary Metal Oxide Semi-Conductor (CMOS) imager with column-feedback soft-reset for imaging under ultra-low illumination and with high dynamic range.
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2. Background Art
Image sensors are devices capable of converting an image into a digital image. Image sensors are also referred as “silicon film” or “silicon eyes”. These devices are made of silicon since silicon has the properties of both being sensitive to light in the visible spectrum and being able to have circuitry integrated on-board.
A CMOS imager is an image sensor made from silicon. CMOS imagers include an array of photo-sensitive diodes, one diode within each pixel. FIG. 1 illustrates a CMOS imager array. Each pixel 100 in a CMOS imager has a radiation sensitive element 110 with each radiation sensitive element connected to an amplifier 120. A CMOS pixel which converts an optical image into an electronic signal with an arrangement of having an amplifier attached to each radiation-sensitive element is called an “active pixel”.
The active pixels in a CMOS imager can be arranged in a matrix form and be utilized to generate video signals for video cameras, still photography, or anywhere incident radiation needs to be quantified. When an incident radiation interacts with radiation sensitive element 110 in a CMOS imager, charge carriers are liberated and can be collected for sensing. The number of carriers generated in pixel 100 is proportional to the amount of the incident light impinging on the radiation sensitive element and the sensitivity of radiation sensitive element to light. The electronic signal generated by pixel 100 in a CMOS imager is then read directly on an x-y coordinate system.
FIG. 2 is an illustration of a schematic circuit diagram of an active pixel 200 of a conventional CMOS image sensor with a photodiode 220 as a radiation-sensitive element. Photodiode 220 comprises of a p-n junction diode, wherein the p-doped junction side 210 is connected to a negative bias voltage Vsub at 296. The n-doped junction side 230a is connected via connection 230 to drain terminal 230c of reset transistor 240 and to gate terminal 230b of a charge sensing transistor 260. Source terminal 270 of reset transistor 240 is biased to a positive reference bias voltage VREF at 292. Gate terminal 242 of reset transistor 240 is connected to a common row reset line 250.
Source terminal 255 of sense transistor 260 is connected to reference bias voltage VDD at 294. The drain terminal of sense transistor 260, and the source terminal of row select transistor 280 are connected to each other at 265. Row select transistor 280 has its gate terminal 282 connected to a row select signal line 285, and its drain terminal 290 connected to a signal line at 298. Identical pixels represented by active elements 220, 240, 260, and 280 are laid out in the form of a matrix pattern comprising of rows and columns to form the CMOS image sensor array.
FIG. 3 is an illustration of a schematic circuit diagram of an active pixel of a conventional CMOS image sensor operating under ultra-low illumination (e.g., taking photograph from outer space or at night without any flash light). The radiation-sensitive element is a sense element 305 comprising a p-n junction, wherein the p-doped junction side 310 is exposed to low optical radiation 315 and the n-doped junction side 330a is connected via connection 330 to drain terminal 330c of reset transistor 340 and to gate terminal 330b of a charge sensing transistor 370. Source terminal 360 of reset transistor 340, and source terminal 365 of a charge sensing transistor 370 is biased to a positive reference bias voltage VDD at 301. Gate terminal 355 of reset transistor 340 is connected to a common row reset line at 350. A row select transistor 380 has its gate terminal 382 connected to a row select signal line 385, and its drain terminal 387 connected to a column signal bus 390.
When p-doped junction side 310 of sense element 305 is exposed to the optical radiation of low illumination 315, the p-doped junction side 310 releases electrons 320. The released electrons 320 flow towards the n-doped junction side 330a of sense element 305. The number of electrons which are released and flow depend upon the intensity of the optical radiation 315, and the sensitivity of p-dope junction side 310 to the optical radiation. This process of conduction can be compared to a p-n junction diode or to a photodiode.
The charge from n-doped side 330a flows to gate terminal 330b of sensing transistor 370. The positive voltage at gate terminal 330b makes sensing transistor 370 conduct a reference bias voltage VDD 301. When sensing transistor 370 is selected to conduct, the charges are passed to source terminal 375 of row select transistor 380. The row select line 385 has a positive voltage when a particular row is selected to sense. When row select line 385 has a positive voltage, gate terminal 382 of row select transistor 380 is turned “on” to make row select transistor 380 conductive. When row select transistor 380 is selected to conduct, the charges from source terminal 375 are passed on to drain terminal 387 of row select transistor 380, and finally is read-out by column bus line 390.
By applying a positive reset voltage to the row reset line 350, all the sense elements 305 in that row are reverse biased to the reference bias voltage VDD 301. When the reset voltage is removed while all sense elements 305 are exposed to an optical radiation, the charge stored in the sense elements 305 in the respective row decreases due to the induced leakage (photo) current generated by the photo-induced electron-hole pair causing the voltage at gate 330b of the charge sensing transistor 370 to decrease proportionately. By applying a row select voltage to gate 382 of row select transistor 380, a signal representing the voltage at gate 330b (and therefore also the charge stored in SENSE element 305) can be read out column-wise via signal lines 390 connected to drain terminal 387 of each row select transistor 380 in a respective column.
After processing the signals from all the active pixels in a CMOS imager array, the final image is reproduced in a digital form. FIG. 4 is an illustration of a CMOS imager capturing a image under ultra-low illumination. Image 401 is captured using CMOS imager 402 under low illumination or low intensity of light, especially when capturing images from space or places were there is no resource of providing a flash light over the object to capture good quality image.
CMOS imager 402 converts the optical radiation exposed on it into electrical signals and processes it to produce digital image 403. The digital image 403 is not an exact replication of the actual image 401. The digital image 403 does not have the exact range of illumination which is present in the original image 401. The digital image produced by a CMOS imager is dull or dark, and with very low intra-scene contrast. A CMOS imager is unable to capture any image under low illumination with high dynamic range and high intra-scene contrast.
Characteristic Analysis of a CMOS Imager:
A CMOS imager experiences some unwanted electrical signals which interfere with the image being read and transferred. These unwanted electrical signals which interfere with a CMOS imager are called “read noise” or “temporal noise”. Read noise occurs randomly and is generated by the basic noise characteristics of electronic components in a CMOS imager circuit. This type of noise can be compared to a disturbance like the “snow” in a bad TV reception.
To capture an image using a CMOS imager with high intra-scene contrast and wide dynamic rage under ultra-low illumination, the noise level should be low and the full-well value must be high with high quantum efficiency. The full-well value defines the maximum amount of charge (photons) an individual pixel can hold before saturating. Low noise is achieved with photogate CMOS active pixel sensor (APS), but at the cost of greater reduced quantum efficiency and reduction in full-well.
Low noise can be also achieved with pinned-photodiode (PPD) APS, but PPD APS has very low full-well and a poor quantum efficiency. Furthermore, it requires complicated processing and has great difficulty operating under an advanced (deep sub-micron) process. Photodiode APS is most suited for advance sub-micron process, however, photodiode APS has noise value high when achieving a high full-well, which is not suited for high quality imaging.
In order to capture scenes with high intra-scene contrast under a low-illumination with large range of illumination in the final image, the signal-to-ratio at low-light level of a CMOS imager has to be maximized along with the increase in the saturation signal level value of the CMOS imager. The signal-to-ratio at low-light level (SNR) is governed by the following equation:SNR=(QE/RN)  (1)where QE is the quantum efficiency and RN is the read noise. The quantum efficiency, QE, is defined as the ratio between the number of generated electrons and the number of impinging photons and the read noise, RN, is obtained by the root mean square (RMS) value of consecutive samples of the output voltage for one pixel. Thus, to achieve high quality imaging at low-light-level, quantum efficiency (QE) has to be increased simultaneously with a reduction in read noise (RN).
On the other hand, to achieve imaging with high intra-scene contracts, the saturation signal level(full-well) value of the CMOS imager has to be increased. Currently, photodiode-type CMOS imager do not allow this. For a typical CMOS imager, the read noise level RN remains high between 25 and 70 electrons, and the full-well value only about 70,000 electrons at a very moderate low read noise level of 25 electrons. However, the full-well can be increased to 1,000,000 electrons, but this would increased the noise to about 100 electrons. The increased value of noise when full-well is increased is unacceptable to achieve high quality imaging. The main reason for the increase in noise is that photodiode active pixel read noise is governed by the sense node reset noise. Sense node reset noise can be expressed by the following equation:Qnoise=sqrt(kTCD)  (2)where Qnoise is the uncertainty on the charge stored on the capacitor, k is Boltzman's Constant, T is the absolute temperature and CD is the sense node capacitance value. Thus, to achieve a low noise level, CD value should be low. But reducing the value of CD to a lower level would reduce the full-well value.
Hence reducing read noise by reducing CD is a conflict with achieving large full-well which demands a large CD value. This conflicting requirement on the sense node capacitance size CD is one of the main limitations of a CMOS imager in simultaneously achieving large full well and low capacitance. All the above discussed technical limitations make a CMOS imager impossible to achieve high intra-scene contrast under a low-illumination with a large range of illumination in the final image.